a) Field of the Invention
This invention relates to a slurry mixing feeder for feeding a slurry, which contains at least a dispersion of fine abrasive particles and a solution of one or more additives at a desired mixing ratio, to a chemical mechanical polishing machine which with high precision, polishes and flattens a surface of a substrate such as a wafer, and also to a slurry mixing and feeding method making use of the slurry mixing feeder.
b) Description of the Related Art
Keeping in step with the move towards high-integration, high-performance LSIs in recent years, there are increasing interests in the chemical mechanical polishing (CMP) method as a processing method for flattening surfaces of substrates, such as wafers, with high precision. In this polishing method, a slurry is used. This slurry is prepared by mixing a solution, which contains a surfactant and a noxidizing agent for promoting chemical action, such as aqueous hydrogen peroxide or iron nitrate, (hereinafter called an xe2x80x9cadditive solutionxe2x80x9d), as needed depending upon a material to be polished, with a dispersion of fine abrasive particles (hereinafter called a xe2x80x9cstock slurryxe2x80x9d) The stock slurry can be obtained by dispersing polishing abrasive particles, which are composed of fine particles of silica, alumina, zirconia, manganese dioxide, ceria (cerium oxide) or the like, in an aqueous alkaline solution of potassium hydroxide, ammonia or the like or in surfactant-containing water. Therefore, the slurry is a dispersion of polishing abrasive particles and additives, and is used in actual polishing. Excellent polishing of a substrate is achieved owing to the combination of chemical action, which takes place between the additive solution in the slurry and the substrate, and mechanical action between the polishing abrasive particles in the slurry and the substrate.
Upon polishing, for example, a silicon dioxide film (oxide film) as a layer insulation film material on a semiconductor silicon substrate by the above-described chemical mechanical polishing machine, a slurry is used. To prepare this slurry, an aqueous alkaline solution, for example, an aqueous solution of potassium hydroxide is added to a silica-particle-containing stock slurry to improve the dispersion property of the silica particles and also to bring the silica particles into a flocculated state optimal to the polishing. The slurry is fed onto the semiconductor silicon substrate mounted on the chemical mechanical polishing machine, and by the silica particles in the slurry and a polishing pad of the polishing machine, mechanical polishing is then performed to remove the oxide film.
In polishing a tungsten metal film as a conductor material, on the other hand, an alumina slurry is used. This alumina slurry is prepared by adding aqueous hydrogen peroxide as an oxidizing agent to a stock slurry which contains alumina particles. By feeding the alumina slurry onto a semiconductor silicon substrate mounted on a chemical mechanical polishing machine, a chemical reaction is induced between a surface of the tungsten metal film and hydrogen peroxide to form a tungsten oxide film polishing of which is easy. The film formed through the reaction is then mechanically polished by the alumina particles, as polishing abrasive particles, and a polishing pad of the polishing machine to remove unnecessary parts other than conductor portions.
As a method for feeding a slurry to such a chemical mechanical polishing machine as described above, it has been a conventional practice to mix a stock slurry, which contains polishing abrasive particles chosen as desired, an additive solution with a surfactant, an oxidizing agent and the like contained therein, and further, diluting water, which may be used as needed, at a predetermined ratio in advance, and subsequent to temporary accumulation in a storage tank, to feed the mixture (slurry) to the polishing machine. This method is, however, accompanied by a problem in that the slurry cannot be fed adequately in a good form suited for polishing and moreover, at a desired mixing ratio, because after the mixing, that is, during the accumulation in the storage tank, deteriorations occur in the polishing characteristics of the slurry and the dispersion property of the fine polishing particles in the slurry is lowered, both with time, and the method has low flexibility and applicability when changing the mixing ratio of the slurry components. With a view to overcoming the above-mentioned problem, a slurry feeder is proposed, for example, in JP 2000-202774 A. According to this slurry feeder, an aqueous solution of abrasive particles (stock slurry) and an additive solution are combined in a mixer immediately before injection onto a turntable of a polishing machine, and the plural solutions are then fed as a slurry.
According to an investigation by the present inventors, however, the slurry feeder disclosed in JP 2000-202774 A referred to in the above has been found to involve problems to be described hereinafter. In the slurry feeder, the mixing accuracy of a slurry relies only upon flow meters and constant flow-rate valves openings of which are feedback controlled by the flow meters. In view of the accuracy of the flow meters, substantial errors occur at the flow meters especially in a low flow-rate range. At the constant flow-rate valves, on the other hand, there is a potential problem of blocking with the stock slurry. In some instances, this construction may not be able to adequately feed a slurry of a specific mixing ratio suited for desired processing. In the above-described conventional apparatus, plural solutions are fed to the apparatus by pumps, respectively. According to an investigation by the present inventors, it has also been found that the system has difficulty in maintaining the mixing accuracy of a slurry at high level because pulsation (pressure fluctuations) of the pumps employed in the conventional apparatus adversely affects the maintenance of constant flow rates by the constant flow-rate valves. Further, the above-described conventional apparatus is not equipped with any cleaning means for the part where mixing is performed. If blocking takes place at the internal piping of the apparatus due to settling or flocculation of fine particles in the slurry while a mixed solution is not used, the fine particles so settled or flocculated cannot be eliminated. A problem is believed to remain unsolved in accurately maintaining a liquid mixing ratio especially in an initial stage after resumption of slurry feeding.
An object of the present invention is, therefore, to provide a slurry mixing feeder, which can adequately feed to a chemical mechanical polishing machine a slurry at a desired flow rate suited for intended processing, at a high-accuracy mixing ratio and in a good form free of deteriorations.
Another object of the present invention is to provide a slurry mixing and feeding method, which can adequately feed to a chemical mechanical polishing machine a slurry at a desired flow rate suited for intended processing, at a high-accuracy mixing ratio and in a good form free of deteriorations.
A further object of the present invention is to provide a slurry mixing feeder, which can maintain a liquid mixing ratio of a slurry at high accuracy even in an initial stage when feeding of the slurry is resumed subsequent to a temporary stop.
The above-described objects can be achieved by the present invention to be described hereinafter. Described specifically, the present invention, in one aspect thereof, provides a slurry mixing feeder for feeding a slurry to a chemical mechanical polishing machine, said slurry containing liquids at a desired mixing ratio, said liquids including at least a dispersion of fine abrasive particles and a solution of an additive, comprising suction ports for sucking the liquids, respectively, a number of the suction ports corresponding to that of the liquids; a discharge port for feeding the slurry to the chemical mechanical polishing machine; feed pumps arranged in feed lines for the respective liquids, said feed lines extending from the individual suction ports to the discharge port, for sucking the individual liquids in specific amounts to give the mixing ratio and delivering the thus-sucked liquids toward the discharge port; and dampers and pressure-regulated restrictors arranged in combination in the feed lines on delivery sides of the feed pumps, respectively.
The slurry may preferably comprise the dispersion of the fine abrasive particles, the solution of the additive and pure water at a desired mixing ratio. Preferably, the slurry mixing feeder may further comprise a means for circulating at least the dispersion of the fine abrasive particles, out of the individual liquids sucked through the suction ports, at a flow rate and pressure equal to or higher than specific rate and pressure at which the dispersion of the fine abrasive particles is consumed at the chemical mechanical polishing machine and a controller for correcting a delivery rate of at least the dispersion of the fine abrasive particles from its corresponding feed pump on a basis of values obtained by continuously measuring pressure fluctuations of the a circulating flow of the dispersion of the fine abrasive particles. Also preferably, the slurry mixing feeder may further comprise a feed line for feeding pure water to the feed line for the dispersion of the fine abrasive particles such that the feed line for the dispersion of the fine abrasive particles can be cleaned with the pure water. The feed pumps may preferably be tubular diaphragm pumps. It may also be preferred that the slurry mixing feeder further comprises a means for transmitting information on a liquid mixing ratio of the slurry, said liquid mixing ratio being desired by the chemical mechanical polishing machine, from the chemical mechanical polishing machine to the feed pumps.
The present invention, in another aspect thereof, also provides a slurry mixing and feeding method for feeding, to plural chemical mechanical polishing machines, slurries desired by the polishing machines, respectively, which comprises connecting slurry mixing feeders of one of the above-described embodiments to the individual chemical mechanical polishing machines, respectively, such that liquids comprising at least a dispersion of fine abrasive particles and a solution of an additive are fed in a parallel manner to the individual chemical mechanical polishing machines via their corresponding slurry mixing feeders. In this slurry mixing and feeding method, it is particularly preferred to use slurry mixing feeders each of which circulates at least the dispersion of the fine abrasive particles (stock slurry) through a pump and is equipped with a controller constructed such that the delivery rate of the stock slurry from a feed pump is corrected on a basis of values obtained by continuously measuring pressure fluctuations of the a circulating flow of the stock slurry.
According to the slurry mixing feeder of the present invention, a slurry formed of plural liquids, which include a stock slurry with fine abrasive particles dispersed therein, can be adequately fed in a deterioration-free good form to the chemical mechanical polishing machine while feeding the individual liquids at desired delivery flow rates and maintaining a high-accuracy mixing ratio.
According to the slurry mixing and feeding method of the present invention, the above-described excellent advantageous effects can be obtained even when plural liquids, which include a feed slurry with fine abrasive particles dispersed therein, are mixed and fed in a parallel manner to plural chemical mechanical polishing machines.
When the slurry mixing feeder further comprises the feed line for feeding pure water to the feed line for the dispersion of the fine abrasive particles such that the feed line for the dispersion of the fine abrasive particles can be cleaned with the pure water, the liquid mixing ratio of a slurry can be maintained highly accurate even in an initial stage after resumption of feeding of the slurry subsequent to a stop of operation. In this case, use of an automated cleaning system makes it possible to provide a slurry mixing feeder maintenance of which is easy.